Astronomers have determined that TRAPPIST-1 d, a rocky exoplanet orbiting a nearby star, does not possess an Earth-like atmosphere. The findings, published in The Astrophysical Journal on August 13, are based on observations from NASA’s James Webb Space Telescope.
TRAPPIST-1 d is one of seven known planets in the TRAPPIST-1 system, located about 40 light-years away. Discovered in 2017 using data from NASA’s retired Spitzer Space Telescope and other observatories, the system holds the record for most Earth-sized rocky planets found around a single star.
The planet is similar in size to Earth and lies within its star’s habitable zone—where liquid water could exist on its surface. However, the latest study shows it lacks molecules common to Earth's atmosphere such as water vapor, methane, or carbon dioxide.
“Ultimately, we want to know if something like the environment we enjoy on Earth can exist elsewhere, and under what conditions,” said Caroline Piaulet-Ghorayeb of the University of Chicago and Trottier Institute for Research on Exoplanets (IREx) at Université de Montréal, lead author of the study. “While NASA’s James Webb Space Telescope is giving us the ability to explore this question in Earth-sized planets for the first time, at this point we can rule out TRAPPIST-1 d from a list of potential Earth twins or cousins.”
Using Webb's near-infrared spectrograph instrument, researchers did not detect signs of an atmosphere similar to Earth's. Still, several possibilities remain open regarding what kind of environment exists on TRAPPIST-1 d.
“There are a few potential reasons why we don’t detect an atmosphere around TRAPPIST-1 d,” Piaulet-Ghorayeb explained. “It could have an extremely thin atmosphere that is difficult to detect, somewhat like Mars. Alternatively, it could have very thick, high-altitude clouds that are blocking our detection of specific atmospheric signatures—something more like Venus. Or, it could be a barren rock, with no atmosphere at all.”
TRAPPIST-1 d completes an orbit around its host star every four days due to its proximity. Despite being close to its sun compared to Earth’s distance from our Sun, its cooler red dwarf star means temperatures might still allow for liquid water under certain conditions.
The parent star itself poses challenges for planetary atmospheres. Red dwarf stars such as TRAPPIST-1 are known for emitting flares and radiation capable of stripping away atmospheres from their closely orbiting planets.
“Webb’s sensitive infrared instruments are allowing us to delve into the atmospheres of these smaller, colder planets for the first time,” said Björn Benneke of IREx at Université de Montréal and co-author of the study. “We’re really just getting started using Webb to look for atmospheres on Earth-sized planets, and to define the line between planets that can hold onto an atmosphere, and those that cannot.”
Scientists continue observing other planets in this system using Webb. While two inner worlds have already been deemed inhospitable due to their proximity to energetic stellar eruptions from their host star, four outer siblings—planets e through h—may have better chances of retaining atmospheres because they receive less intense radiation. Detecting any possible atmospheres there will be more challenging due to their greater distance and colder temperatures.
“All hope is not lost for atmospheres around the TRAPPIST-1 planets,” Piaulet-Ghorayeb said. “While we didn’t find a big, bold atmospheric signature for planet d, there is still potential for the outer planets to be holding onto a lot of water and other atmospheric components.”
The James Webb Space Telescope project involves collaboration between NASA and international partners ESA (European Space Agency) and CSA (Canadian Space Agency). The research received funding support from agencies including the Canadian Space Agency; Natural Sciences and Engineering Research Council; Trottier Family Foundation; Brinson Foundation; Fonds de recherche du Québec—Nature et technologies; iREx; SNSF; Sorbonne University; BELSPO BRAIN; InitiaSciences; TESS Guest Investigator Program; Swiss National Science Foundation; NRC Canada.
“Strict Limits on Potential Secondary Atmospheres on the Temperate Rocky Exo-Earth TRAPPIST-1 d.” Piaulet-Ghorayeb et al., The Astrophysical Journal Aug. 13 2025.